The invention relates to a new compound of a calcium trisulfoaluminate base and also to a process for its manufacture.
The manufacture of ettringite (calcium trisulfoaluminate) starting with aluminum sulfate is already known.
In the French Pat. No. 72/40,247, applied for by the applicants on Nov. 13, 1972, there is described a process for the manufacture of a fine, white charge for industry, as well as the ettringite essentially obtained by the process. In accordance with the cited patent, the process for the preparation of the ettringite or calcium trisulfoaluminate (3 CaO. Al.sub.2 O.sub.3. 3 CaSO.sub.4. 32 H.sub.2 O) is characterized by the simultaneous hydration of a mixture of an accurate stoichiometric composition at a temperature from about 20.degree. to about 90.degree. C of calcium aluminate, calcium sulfate which is as white as possible, and water in at least a quantity for a stoichiometric proportion for the reaction and at the maximum such that after the reaction, a product containing 5% by weight of ettringite (in dry form) and 95% (by weight of water), is obtained, the stoichiometry being defined by at least one of the following reactions:
______________________________________ CaO. Al.sub.2 O.sub.3 + 2(CaO. H.sub.2 O) + 3(CaSO.sub.4. 2 H.sub.2 O).fwdarw. (CaO).sub.3 Al.sub.2 O.sub.3. 3CaSO.sub.4. 32H.sub.2 O (ettringite) (CaO).sub.3 Al.sub.2 O.sub.3 + 3(CaSO.sub.4. 2H.sub.2 O) + 26 H.sub.2 O .fwdarw.1 ettringite CaO. (Al.sub.2 O.sub.3).sub.2 + 5(CaO. H.sub.2 O) + 6(CaSO.sub.4. 2H.sub.2 O) + 47 H.sub.2 O .fwdarw. 2 ettringite (CaO).sub.12.(Al.sub.2 O.sub.3).sub.7 + 9(CaO. H.sub.2 O) + 21(CaSO.sub.4. 2H.sub.2 O) + 173 H.sub.2 O .fwdarw.7 ettringite. ______________________________________
The obtained charge principally consists of ettringite (calcium trisulfoaluminate).
It is indicated in the above mentioned patent, that the process described there does not only allow the production of all forms such as dry powder, solutions, or suspensions of concentrations or contents, practically variable at will, but also allows the utilization, as the starting materials, of different industrial compounds such as, for example, aluminous cements, unburnt cements, calcium sulfates, as well as hydraulic binders which do not exactly possess the properties of the cements which, normally, they are destined to form.
It is known that the calcium silicates -- providing the utilized water of a sufficient quantity -- become hydrated by furnishing a mixture of hydrated calcium silicates, hydrated lime and eventually of silica. The hydrated silicates referred to as tobermorites are present in the form of hydrates of the general formula x CaO. y SiO.sub.2. z H.sub.2 O, with x/y between from about 0.4 to about 3 and z/y between from about 0.5 to about 6. The Afwilite of the chemical formula of 3 CaO. 2 SiO.sub.2. 2 to 4 H.sub.2 O is present as a particular type of tobermorite.
The present invention utilizes this last hydration reaction as a source of lime for the manufacturing procedure, beginning with hydrated aluminum sulfate as starting material, of a compound essentially of calcium trisulfoaluminate.